1. Field of the Invention
The present invention relates to an optical heterodyne measuring apparatus for effecting a measurement based on a difference between beat frequencies of reference and measuring beams, and more particularly to improvements in such an optical heterodyne measuring apparatus wherein a laser beam or beams is/are transmitted through a fixed-polarization plane optical fiber, to a measuring portion of the apparatus.
2. Discussion of the Prior Art
There is known an optical heterodyne measuring apparatus, wherein two linearly polarized reference beams having different frequencies, and two linearly polarized measuring beams identical with the reference beams are obtained from a laser beam or beams which is/are transmitted to a measuring portion of the apparatus through a fixed-polarization-plane optical fiber. In this apparatus, a measurement is accomplished based on a difference between the beat frequency of the reference beams, and the beat frequency of the measuring beams which varies depending upon a parameter of a subject which is to be measured. Described more specifically, when one of the two linearly polarized measuring beams is incident upon the subject, that measuring beam is subject to a frequency shift due to the Doppler effect. As a result, the beat frequency of the two measuring beams consisting of the above-indicated one beam reflected by the subject and the other beam also varies by an amount of the Doppler shift of the frequency of the beam incident upon the subject. Based on the difference between the varying beat frequency of the measuring beams and the fixed beat frequency of the reference beams, a desired parameter of the subject such as the surface roughness, surface profile and displacement may be measured. In this type of measuring apparatus, the laser beam or beams is/are propagated from a laser source to the measuring portion through the fixed-polarization-plane optical fiber, whereby the measuring portion is not affected by the heat generated by the laser source, and can be made relatively small-sized and compact.
Generally, the laser source produces two linearly polarized laser beams having mutually perpendicular polarization planes (similar to the two reference or measuring laser beams), or a single linearly polarized laser beam. Where the two linearly polarized laser beams are produced by the laser source, these two laser beams are split by a beam splitter or other means into the two reference beams and the two measuring beams. Where the single laser beam is produced by the laser source, the laser beam is incident upon an optical frequency shifter, to provide two laser beams having different frequencies. These two laser beams are then incident upon a beam splitter or other means, to provide the two reference beams and the two measuring beams.
In such an optical heterodyne measuring apparatus, it is known to detect a change in the beat frequency of the measuring beams, such that the two measuring beams whose beat phases are shifted from each other by 180.degree. are received by respective photosensors, and the electric signals produced by the photosensors are applied to a differential amplifier, so that the detection of the change in the beat frequency is made based on an output of the differential amplifier. This arrangement permits improved accuracy of measurement of the apparatus, since the arrangement is effective to reduce an influence of the noise components included in the intensity amplitudes of the measuring beams.
However, the optical heterodyne measuring apparatus using a differential amplifier which receives the electric signals from the photosensors as described above also suffers from the external disturbances such as heat and vibrations to which the optical fiber is exposed, which disturbances may cause up to about several tens of nanometers of error in the measurement of a surface roughness, for example.
The causes for the above problem will be considered, in the two cases where different types of laser source are used. In one case, the laser source produces two linearly polarized laser beams whose polarization planes are perpendicular to each other and whose frequencies are different from each other. In the other case, the laser source produces a single linearly polarized laser beam.